There is an increasing interest in electric light bulbs which do not make use of incandescent filaments, since filament-based light bulbs are considered to be inefficient and energy hungry. Indeed, recent legislative changes mean that traditional incandescent light bulbs are being phased out in many parts of the world. One existing replacement for the incandescent light bulb is the compact fluorescent tube bulb.
Solid state lighting (SSL), for example light emitting diode (LED) or organic light emitting diode (OLED) based retrofit lamps, offer superior performance over compact fluorescent lamp (CFL) based retrofit lamps in terms of efficiency, instant light output, light quality, and lifetime. The main barrier to penetrate the market is product cost, since the shop price of today's LED-based lamps can be up to 10 times of CFL lamps.
A key element of an LED lamp assembly is the LED light source. The luminous efficiency, measured in lumen per watt, has been improved significantly over the last 10 years, and continues to increase further to levels of 250 lm/W for white light LEDs, with potential for further improvement.
Another strong advantage of using LED light sources is that they offer superior lifetime since the only failure mode is a slow depreciation of the outcoupling optics of the light source.
SSL lamp assemblies are generally operated using mains or power line electrical supplies, which provide an AC voltage, typically having a level of 110/120V or 230V/240V, at frequencies of between 50 and 60 Hz. This alternating nature of supply voltage consequently causes the power into the SSL lamp assembly to be inherently time dependent. The power typically changes between zero and multiples of the average system power within one power cycle.
The SSL device needs to be supplied with a DC drive signal. Any fluctuation of the DC drive signal can lead to visible effects such as flickering and to a degradation of the SSL efficiency due to so called droop effects at increased current levels.
Whenever the instantaneous input power is higher than the instantaneous output power the excess power has to be stored inside the power converter. Whenever the instantaneous input power is lower than the instantaneous output power the missing power has to be delivered from inside the power converter. Therefore, power converters usually include an electrical storage element.
Electrical energy can be stored as current using inductors or as voltage using capacitors. To store electrical energy of reasonable amount in the time domain of higher than milliseconds, inductive storage elements tend to become extremely large and bulky. Power converters typically use capacitors for energy storage.
In one previously-considered design, a single stage is provided which includes one switching element. The switching element being a MOSFET or a bipolar device or any other device is capable of switching electrical magnitudes at reasonably high frequencies.
Typical example single stage converters include flyback converters, buck converters or buck/boost converters. If a single stage converter is used said capacitive storage element can be connected to either the input or the output of the converter.
If the storage element is placed at the input, mains current is drawn only in a very short period of time causing large distortion of mains current which has to be filtered to comply with legal standards. Further the capacitive storage element has to withstand very high voltages typically above the peak of the mains voltage at the typical ambient temperature that occurs inside the SSL lamp assembly.
Such capacitive storage elements are typically aluminum electrolytic capacitors with a wet electrolyte which tends to slowly evaporate or diffuse causing degradation of the device especially at increased temperature levels. Such devices are expensive, sensitive to lifetime limitations and bulky.
If the storage element is placed at the output of the power converter, it is inherently placed in parallel to the SSL device. SSL devices inherently produce a very low incremental resistance. Consequently the capacitance of the energy storing device must be very large in order to achieve reasonable filtering of the current fluctuation into the SSL device. Typically capacitances substantially higher than 100 uF are needed. In many cases capacitances at 1000 uF or higher are needed.
Such capacitive storage elements are typically aluminum electrolytic capacitors with a wet electrolyte which tends to slowly evaporate or diffuse causing degradation of the device especially at increased temperature levels. Such devices are expensive, sensitive to lifetime limitations and bulky.
One previously-considered power converter topology has a common reference potential of input, output and power switch. Such a topology offers the advantage of simple control but has a significant disadvantage in that the output voltage is higher than the input voltage. These topologies are also referred to as boost circuits. To be used inside an SSL lamp assembly the output voltages become higher than the peak mains voltage yielding output voltages as high as 400V and above. The required capacitors are bulky, costly and strongly limited in their lifetime under elevated temperature conditions.